The present invention relates to speed governors suitable for use in controlling the speed of high speed turbines and other fluid driven motors, and especially those used in down-hole drilling applications and the like.
It is a particular problem with down-hole drilling that the material being drilled through can be subject to frequent and/or large changes in hardness and/or other properties resulting in significant changes in drilling resistance. When there is a more or less sudden reduction in drilling resistance, which is especially large when the drilling bit lifts off from the bottom of the borehole for any reason, there is a tendency for the motor to speed up veiy quickly and dramatically—typically in as little as 20 to 100 milliseconds. In the past, prevention of such over-speeding has been attempted by very careful control of the drilling operation to try to minimize any possible lift off, and/or to use braking systems such as eddy current brakes. Conventional speed governors are generally of the centrifugal clutch type in which weights are forced radially outwardly against a spring resistances. An increase in speed increases the centrifugal forces acting on the weights and forces a brake shoe against a brake drum. In the case of both such conventional speed governors and eddy current brakes there is however a significant loss in efficiency since they apply some degree of braking at all times. Furthermore when increased braking is applied, the flow of motive fluid through the motor or turbine is significantly reduced thereby building up fluid pressure behind the turbine.
It is an object of the present invention to avoid or minimize one or more of the above disadvantages or problems.